This invention relates to rotary screens which are used for separating liquids from solids, for the purification of liquids or the recovery of solids from slurries.
Such screens are typically used in industrial processes, for example for the removal of waste products from effluents before discharging the effluents into drainage systems and for recovery of solids such as wood pulp from industrial slurries, and also in sanitary engineering installations, such as sewage filtration plants.
The rotary screens are in the form of drums having perforated circumferential walls for the passage of liquid through them, and mounted for rotation about axes which are substantially horizontal. It is well known to construct such rotary screens for "dry" operation, that is operation in which a slurry to be screened is fed into the inside of the screen by feeding means and the liquid from the slurry is collected from the outside of the screen, the screen itself not being immersed in the slurry. However, in many circumstances it may be preferable to provide a rotary screen which is capable of operating partially immersed in the slurry being screened.
Attempts have been made in the past to design a rotary screen which is suitable for such operation, and reference is made to U.S. Pat. No. 2,664,204 of A. T. Hurter and U.S. Pat. No. 3,979,289 of M. J. Bykowski and L. Ewing, which illustrate such screens. The screens shown in the patent specifications referred to are provided with screening walls formed of a perforated sheet material, but more recent design has favoured the use of spaced-apart wires of wedge-shaped cross-section, the wires generally extending longitudinally of the drum and being interlinked by supporting hoops extending circumferentially of the drum. Such construction is shown, for example, in British patent specification No. 2,076,307 of Alchaldean International Pty Limited.
In most applications of a rotary screen, the solids to be separated from liquids by the screen consist at least partially of fibres. When such fibres are screened by a rotating screen, they tend to wrap around solid portions of the screen wall which are moving relative to the slurry. Such fibres can accumulate to the extent that they block the apertures in the screen and prevent effective filtration. This problem is particularly acute when the screeen is partially immersed in the slurry so that the slurry is not caused to flow relative to the screen. The efficiency of known screens in which the screen wall is formed by perforated sheet material or of wedge wires which extend longitudinally of the screen is very poor when the screens are operated partially immersed in the slurry.
The above-mentioned patent specifications show conventional kinds of mounting and driving means for rotary screens, in which the screen is at least mounted, and sometimes driven by, means which are disposed partially or wholly below the lowest point of the rotation axis of the screen. Where the screen is to be operated partially immersed in the slurry being screened, such mounting and driving means must be either protected from the slurry or designed so that they are able to operate in the slurry. These design requirements substantially increase the costs of designing screens and manufacturing screens. The preferred embodiment described in this specification will reduce this problem by providing alaternative driving and supporting means for a rotary screen.